1. Field of the Invention
This invention relates generally to a novel audiometric device and more particularly is concerned with the field of acoustic impedance audiometry.
2. Description of the Prior Art
The conventional method of testing hearing has traditionally utilized a machine capable of producing an audio output at a set frequency, with the output variable in frequency and loudness. A skilled operator is required to select the proper sequence of tones, at different frequencies and sound amplitudes, which are transmitted successively to the ear of a test subject who then indicates, as by depressing a push button switch, whether each sound was or was not heard. The test subject's response is subsequently analyzed to compile a profile of hearing ability. This method is disadvantageous because it is time consuming, requiring a number of test "runs" to collect sufficient data. Furthermore, a skilled operator must be provided to properly perform the test. Such a test may produce inaccurate data because the tested subject's response can be highly subjective due to the way in which sounds are perceived. Because the subject of such a test necessarily must actively participate in the test, this method cannot be used to evaluate the hearing of a patient who is unable or unwilling to respond, such as a young child. Although such conventional hearing test methods are normally conducted with the test subject in an acoustically isolated enclosure, high ambient noise levels can interfere with the test subject's perception of the test tones and thereby introduce further error into the test results.
In response to the need for a more accurate and reliable method of testing hearing ability, techniques have been developed which measure an involuntary reaction of the tested subject to an applied sound. The test method thus can be made independent of the subject's conscious perception of the sound stimulus, while providing an indication of hearing proficiency. U.S. Pat. No. 4,007,731, to Griffiths et al., for example, describes a testing method which utilizes an observed cause-and-effect relationship between the applied sound stimulus and a change in heart beat, and U.S. Pat. No. 3,799,146, to John, teaches a technique for analyzing brain wave activity to detect the presence of a neural response to an applied sound.
One of the more useful involuntary physiological indications of a reaction to a sound stimulus is that of the intra-aural muscles which are located in the middle ear. These muscles exhibit a characteristic contraction in response to sound stimuli. Such a contraction will in turn cause a change in the tension of the eardrum. This change in eardrum tension produces a change in equivalent volume in the ear canal. When that ear canal is pressure sealed by an ear plug, this change in volume can be measured, and will produce an electrical signal from a transducer exposed to the sealed ear canal. This electrical signal can then be calibrated to provide a measure of the compliance, or flexibility, of the eardrum. The correlation between the presence of an intra-aural muscle reflex and a test subject's hearing proficiency is known in the art. For example, U.S. Pat. No. 3,395,697, to Mendelson, describes a device which detects the presence or absence of a change in compliance in response to an applied stimulus which can be varied in frequency and amplitude. U.S. Pat. Nos. 3,949,735, to Klar, and 4,009,707, to Ward, disclose refinements of this technique of evaluating hearing ability.
While the significance of the intra-aural muscular contraction for diagnosis of hearing deficiencies is thus known in the art, the use of this phenomena has been limited. Known methods for measuring the reflex are designed to discriminate between the presence or absence of a response of the intra-aural muscles. The prior art techniques, however, have not recognized the clinical significance of the time characteristics of this response. No hearing test method has heretofore utilized as a diagnostic parameter the magnitude of the intra-aural muscle response as a function of the time lapse after an applied sound stimulus.
It is therefore a feature of this invention to provide an improved method and apparatus for testing hearing which measures the involuntary response time of the hearing mechanism.
It is another feature of this invention to provide an improved diagnostic method and apparatus to increase the accuracy of a hearing test.
It is another feature of this invention to provide an improved diagnostic method and apparatus which simplifies the procedure required for a hearing test.
It is another feature of this invention to provide an improved diagnostic hearing test method and apparatus which is less dependent on the subjective response of the patient tested.
It is another feature of this invention to provide an improved method for testing hearing which requires a minimum of time to conduct.
It is another feature of this invention to provide an improved method and apparatus for testing hearing which is less dependent on the skills of a trained operator.
It is another feature of this invention to provide an improved method and apparatus for testing hearing which is less subject to error in the presence of ambient noise levels.
It is another feature of this invention to provide an improved acoustic impedance apparatus for testing hearing which is less expensive to produce.
It is an additional feature of this invention to provide a compliance testing method which does not require an air tight seal in the ear.